1. Field of the Invention
This invention relates generally to low dose ion implantation monitoring. More particularly, this invention relates to a computer-implemented reflectance system and method for associating absolute values of reflectance changes over the entire measured spectra to doses of ions implanted in a semiconductor wafer for high resolution ion implantation monitoring in a non-destructive, efficient, accurate, and repeatable manner.
2. Description of the Related Art
In the semiconductor manufacturing industry, certain materials are often doped with impurities to change their properties such as electrical or physical properties during different stages of the semiconductor manufacturing process. These materials may include silicon, germanium, or gallium arsenide. The impurities, i.e., dopants or doping agents, such as B, P, Ga, Ge, F, Si, B11, BF2, Sb, In, As, and H, can be diffused or implanted into the materials. The diffusion process is useful for large-scale applications. The ion beam implantation is presently being utilized in small-scale electrostatic processes.
For different purposes, implant doses may vary from about 1010 ions/cm2 to about 1018 ions/cm2. During an ion implantation process, ions of a doping agent enter a semiconductor material and collide with atoms of the material, causing displacements of the atoms. As a result, the material is damaged or modified in regions implanted, i.e., doped, with the ions. A common practice in the art to remove some of the damage to the crystalline structure is by thermally annealing the material, although part of the material may become amorphous rather than crystalline with a sufficiently high dose.
As is well known in the art, because of the small dimensions and narrow dose tolerances of the devices being created, it is critically important to accurately monitor and/or characterize the ion implant doses. The monitored result or characterization can also be used, for example, to evaluate, analyze, and characterize the electrical and/or physical properties of the semiconductor device and/or material for purposes such as flaw testing.
There are several known methods for monitoring ion implant doses, including sheet resistance based methods and thermal wave based methods. The sheet resistance based methods include, for example, single implant sheet resistance method and double implant sheet resistance method.
The single implant sheet resistance method uses a 4-point probe to measure the sheet resistance of specially prepared and treated silicon test wafers after implantation and activation. Most technologies do not rely on this method for low dose monitoring because of the fundamental difficulties in measuring reproducible sheet resistances in the regime of 100,000 ohm/sq. or more.
The double implant sheet resistance method measures the change in 4-point probe sheet resistance of a previously implanted and activated silicon test wafer that is subsequently damaged from a low dose (second) ion implantation. This method suffers from considerable process complexity that causes major wafer-to-wafer reproducibility problems.
The sheet resistance based methods are well known in the art and thus are not further described herein for brevity. For an exemplary teaching on the sheet resistance based methods, readers are referred to xe2x80x9cAdvances in Sheet Resistance Measurements for Ion Implant Monitoringxe2x80x9d by W. A. Keenan et al., Solid State Tech., June 1985, pp. 143-148.
The thermal wave based methods are currently being used in the semiconductor manufacturing process. By analyzing thermal waves generated in an implanted silicon wafer, this type of methods provides a rather non-destructive way of monitoring ion implants in the wafer. The thermal wave methods are based on the effect that damage to the silicon crystal lattice that takes place during ion implantation increases the thermal wave signal above that of the non-implanted silicon wafer. Exemplary teachings on thermal wave systems and methods can be found in the following U.S. patents: U.S. Pat. No. 4,513,384, titled xe2x80x9cTHIN FILM THICKNESS MEASUREMENTS AND DEPTH PROFILING UTILIZING A THERMAL WAVE DETECTION SYSTEMxe2x80x9d and U.S. Pat. No. 4,750,822, titled xe2x80x9cMETHOD AND APPARATUS FOR OPTICALLY DETECTING SURFACE STATES IN MATERIALS,xe2x80x9d both of which are issued to Rosencwaig and assigned to Therma-Wave, Inc. of Fremont, Calif., U.S.A.; U.S. Pat. Nos. 4,854,710, 4,952,063, and 5,042,952, titled xe2x80x9cMETHOD AND APPARATUS FOR EVALUATING SURFACE AND SUBSURFACE FEATURES IN A SEMICONDUCTORxe2x80x9d and U.S. Pat. No. 5,074,669, titled xe2x80x9cMETHOD AND APPARATUS FOR EVALUATING ION IMPLANT DOSAGE LEVELS IN SEMICONDUCTORS,xe2x80x9d all of which are issued to Opsal et al. and assigned to Therma-Wave, Inc. of Fremont, Calif., U.S.A.
The thermal wave based optical systems and methods utilize laser-induced modulation of the optical reflectance. As such, a thermal wave signal is a modulated reflectance signal. Values of the thermal wave signal thus vary depending upon the type of the doping agent (dopant) used. For example, the thermal wave signal values range from 200 to 10,000 for boron (B) ions and from 500 to 100,000 for heavier ions such as phosphorus (P) and arsenic (As) ions. The thermal wave signal and dose for P and As ion implants have a somewhat one to one correlation at low dose ranges of 1E10 to 3E14 ions/cm2. The thermal wave signal correlates well with dose and threshold voltage at low dose ranges of 1E11 to 1E12 ions/cm2.
It is important to note, although the thermal wave signal depends primarily on implant dose, it can be influenced, to a smaller degree, by other implant parameters such as beam energy, beam current and wafer temperature. According to xe2x80x9cMaterials and Process Characterization of Ion Implantationxe2x80x9d edited by Michael I. Current and C. B. Yarling and published by Ion Beam Press, Autstin, Tex., USA, 1997, pp. 8-12, which is hereby incorporated by reference, the thermal wave sensitivity varies for different penetration depths of ions in silicon. It is also sensitive to channeling and various scanning effects.
Additionally, as discussed heretofore, thermally annealing the wafer may remove some of the undesirable damage to the crystalline structure. This annealing process has the potential to also remove some of the desirable modification thereof, i.e., regions of the crystalline structure modified (patterned) with ion implants, thereby causing an undesirable annealing effect. This undesirable annealing effect may potentially be a problem in thermal wave based systems as semiconductor technologies continue to scale because of the 100% intensity modulated laser beam commonly utilized in these systems. That is, some of the intended modification to the crystalline structure may be undesirably removed by the localized heating of the material, rendering the non-destructiveness of these thermal wave based systems questionable.
The concern of undesirable annealing effect generally applies to dose measurement monitoring systems where wafer temperature is increased during the measuring and/or monitoring process. For example, in U.S. Pat. No. 6,268,916, titled xe2x80x9cSYSTEM FOR NON-DESTRUCTIVE MEASUREMENT OF SAMPLES,xe2x80x9d issued to Lee et al., and assigned to Kla-Tencor Corporation of San Jose, Calif., U.S.A., Lee et al. disclosed how to use heat dissipation characteristics of a semiconductor wafer to measure physical properties thereof. The surface temperature of an area of the semiconductor wafer is increased by heat, which is generated by a pump beam produced by an infrared laser. When the wafer has been doped with a dopant, the heat dissipation characteristics of the wafer at the surface area are dependent upon the dose and the implant profile in the damaged layers in the wafer. The heat dissipation characteristics, in turn, determine the change in the temperature of the wafer surface and the change in the complex index of refraction of the surface. The ellipsometer system disclosed by Lee et al. provides a probe beam for interrogating such changes.
Other non-destructive optical systems and methods that do not rely on the thermal wave principle have been developed for use in measuring, monitoring, analyzing, and characterizing semiconductor substrate materials, particular the surface thereof, and the thin films deposited on the surface of the substrate materials. For example, U.S. Pat. No. 4,766,317, titled xe2x80x9cOPTICAL REFLECTANCE METHOD OF EXAMINING A SIMOX ARTICLE,xe2x80x9d issued to Harbeke et al., and assigned to General Electric Company of Schenectady, N.Y., U.S.A., disclosed an optical reflectance method of determining the degree of amorphism, surface roughness, and presence of a contaminating film on the surface of a SIMOX article. Harbeke et al. teach illuminating the SIMOX surface with light beams of three selected wavelengths: 240 nm, 320 nm, and 367 nm. The reflections of these light beams indicate reflectance changes corresponding to amorphism, surface roughness, and the presence of a surface contaminating film.
In xe2x80x9cAdvanced Methods of Ion Implant Monitoring Using Optical Dosimetryxe2x80x9d by J. R. Golin et al., Solid State Tech., June 1985, pp. 155-163, a prior art optical dosimetry method is disclosed. The method measures the optical transmission through a photoresist-coated glass substrate that has been darkened by exposure to the implant beam. A change in the optical density is related to the implant dose. It is important to note that measurement sensitivity for this method falters in the low dose regime with low dose sensitivity of only xc2x110%, as indicated by the published data. What is more, since it requires special glass substrate on which the photoresist layer is deposited, silicon wafers cannot be used in this method. As such, there is questionable correlation to actual implant conditions, e.g., wafer charging or channeling effects, performed on silicon wafers through gate or screen oxides.
On the other hand, it has been discovered that, in some cases, non-destructive optical methods and apparatuses can be used to test highly doped, xe2x80x9copaquexe2x80x9d silicon wafers, i.e., where silicon crystal is impregnated with high dose impurities such as phosphorous (P) or boron (B). For example, U.S. Pat. No. 5,007,741, titled xe2x80x9cMETHODS AND APPARATUS FOR DETECTING IMPURITIES IN SEMICONDUCTORS,xe2x80x9d issued to Carver et al., and assigned to ATandT Bell Laboratories of Murray Hill, N.J., U.S.A., disclosed a method for detecting small amounts of impurity, i.e., trace interstitial oxygen, in highly doped silicon wafers. The wafers have a doping concentration in excess of 1.0xc3x971018 (1E18) conductivity-determining atoms/cm2. This method uses a carbon dioxide laser or a lead-salt diode laser to form a light beam having a high proportion of its power at an optical frequency capable of being absorbed by the impurity to be measured, i.e., at a single wavelength within the characteristic oxygen absorption band 8.9-9.15 microns. Using the system set up disclosed by Carver et al., small changes of reflectivity due to the presence of an interstitial impurity could be detected by comparing the light reflected from such a surface with light reflected from a semiconductor wafer having a known quantity of such impurity. However, Carver et al.""s method and system is limited to detecting presence of an impurity in highly doped silicon wafers and is incapable of measuring reflectance changes at wavelengths other than one that is within the characteristic absorption band of the impurity to be measured.
What is needed in the art is an optical reflectance system and method for high resolution non-destructive monitoring of low dose ion implantation in an accurate and reproducible manner without suffering from potential undesirable annealing effects even as semiconductor technologies continue to scale.
It is therefore a primary object of the present invention to provide a computer-implemented reflectance method that does not suffer from undesirable annealing effects for accurate and non-destructive low dose ion implantation monitoring, the method including the steps of:
a) providing illumination at all wavelengths (wl) on a silicon or silicon oxide wafer;
b) obtaining non-implanted reflectance measurements (Rref) of the wafer at each of the wavelengths (Rref,wl);
c) obtaining implanted reflectance measurements (Rimp) at each of the wavelengths (Rimp,wl);
d) forming respective non-implanted and implanted reflectance values over the entire measured spectra;
e) comparing non-implanted and implanted reflectance values and determining reflectance changes; and
f) determining a reflectance change index value where said reflectance change index equals       ∑          wl      =      190        1000    ⁢      |          (                        (                                    R                              ref                ,                wl                                      -                          R                              imp                ,                wl                                              )                          R                      ref            ,            wl                              )        |  
such that said reflectance change index correlates to the low dose.
It is also an object of the present invention to provide a computer-implemented reflectance system and corresponding computer program product for accurate and non-destructive monitoring of low dose ion implantation, the system comprising:
a single source for providing visible and invisible lights at a substantially broad range of wavelengths (wl) on a silicon or silicon-oxide wafer;
a spectrophotometer for obtaining non-implanted reflectance measurements (Rref) and implanted reflectance measurements (Rimp) of the wafer at each of the wavelengths, (Rref,wl) and (Rimp,wl), respectively, and outputting those reflectance measurements; and
a computer for analyzing the reflectance measurements, comprising a processor, a memory, and a computer-readable medium carrying instructions executable by the processor, the computer-executable instructions comprise:
program codes for forming respective reflectance values of the non-implanted and implanted reflectance measurements over the entire measured spectra;
program codes for comparing the respective non-implanted and implanted reflectance values and determining reflectance changes;
program codes for determining a reflectance change index value that equals             ∑              wl        =        190            1000        ⁢          |              (                              (                                          R                                  ref                  ,                  wl                                            -                              R                                  imp                  ,                  wl                                                      )                                R                          ref              ,              wl                                      )            |        ,
where the reflectance change index correlates to the low dose of ions implanted in the wafer; and
program codes for providing a graphic user interface environment capable of displaying the reflectance change index values, the ion doses, the correlation thereof, and receiving user input.
Still further objects and advantages of the present invention will become apparent to one of ordinary skill in the art upon reading and understanding the following drawings and detailed description discussed herein. As it will be appreciated by one of ordinary skill in the art, the present invention may take various forms and may comprise various components, steps and arrangements thereof. Accordingly, the drawings are for purposes of illustrating principles and embodiments of the present invention and are not to be construed as limiting the present invention.